High peak power laser systems are very useful in a variety of applications requiring high harmonic generation of visible laser beams, optical parametric generation of near and mid IR laser beams etc. Such applications include, inter alia, material processing, optical memories, optical communication systems and medical systems. Most of these applications require short pulse laser beams having high intensity (peak power) at each pulse.
Attempts to use laser systems based on semiconductor gain medium, such as mode-locked diode lasers, for generation of short laser pulses were limited to relatively low peak power. Therefore, traditionally, non semiconductor laser systems such as broad area lasers (e.g. Ti:Sapphire and Nd:YAG) or doped fiber mode locked lasers are used to produce high intensity laser pulses.
In order to achieve pulsed laser beams having high peak power it is known to utilize mode locked laser systems operating with a multitude (e.g. thousands) of longitudinal laser modes. To this end, mode locking generally refers to the coherent addition of laser modes whose frequencies are spaced by a constant value, creating what is known in the art as “frequency comb”, and where said addition is performed coherently, i.e. in a way that the relative phase between the different modes is kept fixed (i.e. locked).
Known in the art methods of the kind specified utilize spatial overlapping of multiple longitudinal modes propagating along the cavity. Referring to FIG. 1, there is illustrated schematically a typical mode locked laser system 100. System 100 includes a single-resonator optical cavity 110 of an optical length Lopt (being the sum of the lengths times the refraction indices of all the elements along the cavity). The optical cavity 110 is defined by a space between a rear mirror 122 and an output coupler 124 (e.g. mirror). The optical cavity has an optical axis 112 of light propagation, a gain medium (or laser gain source) 130 and a mode locker 126 being located along the optical axis 112.